Study Of Preparation And Characterization Of Microarc Oxidation Coating On Commercially Pure Titanium And Its Alloys

Microarc oxidation (MAO) is an advanced technique of directly situ growth oxide layer on the valve metal surface. MAO have intensively attracted the attention of many domestic and overseas research groups for it can significantly enhance or improve the surface hardness, wear resistance, corrosion resistance, high-pressure and high-temperature impact of the Processed materials, And also it can be used to prepare multifunctional ceramic, such as using as the anti-bacteria, acting as a catalyst and being compatible with the biological or gas sensitive coating.In recent years, many domestic and overseas units have engaged in the study of preparation and characterization of MAO coating on titanium and its alloys, and have achieved certain results. However. MAO is a complex process controlled by multiple factors. Process parameters can bring tremendous impacts on the characteristics of the coating. Moreover, the experimental and theoretical research is far from comprehensive and sufficient. Therefore. further research work on the formation mechanism and their physical and chemical characteristics of MAO coating on the titanium and its alloys are needed to do. In this thesis. MAO technology has been used to prepare MAO coating on TA2 commercially pure titanium and its alloy TC4 by using a constant working voltage. In order to fabricate outstanding MAO coating, the coatings were studied systematically and the effects of deposited parameters on the coating s'properties hve been investigated.We developed a high-power multi-functional MAO power supply system firstly, which can output multi-waves and work with two models. Then we build a high-speed and multifunction control PWM circuit. by using the TIM function of STM32F103 MCU. The anode and cathode voltage and current values can be really collected by 12bit high-speed ADC on chip. On this basis, the MAO system had been built, after that the TA2 commercially pure titanium and TC4 titanium alloy substrate were treated by using this system, and the effect of processing parameters on the coating's growth characteristics and physical and chemical properties have been systematically studied.This thesis consists of three parts. In the first part, when the thickness, film bonding strength and corrosion resistance had been defined as indicators of evaluating the coating's performance, the optimized treatment solution was finally selected by Initial selection,Filter and Optimization design of the electrolyte composition. For TA2 commercially pure titanium, the optimal parameters of treatment solution is: 10g·L-1 Na2SiO3 and 15g·L-1 Na3PO4; for TC4 Titanium alloy, the optimal parameters of treatment solution is: 10g·L-1 Na2SiO3. 5g·L-1 Na3PO4 and 4g·L-1 NaOH. The TA2 commercially pure titanium and TC4 titanium alloy substrate were treated by MAO in the optimized treatment solution, and the amplitude,frequency and duty cycle of voltage pulse on the coating thickness, phase composition, surface morphology, micro hardness, film-binding strength and corrosion resistance have been detailed studied. Moreover, the growth process of the coating has been analyzed. The results showed that:as the amplitude of voltage pulse increased, the coating thickness,micro-pores' size, film binding strength, surface roughness and the relative content of rutile TiO2 are increased gradually While the micro-pores' density and the relative content of anatase TiO2 gradually decreased. The Corrosion resistance of the coating increased firstly and then rapidly decreased. As the frequency of voltage pulse increased, the coating thickness, micro-pores' size. surface roughness. film binding strength, corrosion resistance and the relative content of rutile TiO2 reduced gradually while the micro-pores' density and the relative content of anatase TiO2 increased gradually. As the duty cycle of voltage pulse increased. the thickness of the coating, the micro-pores' size and surface roughness are increased gradually, while the density of micro-pores and the relative content of anatase TiO2 are gradually decreased. At the same time, the film binding strength and Corrosion resistance of the coating are firstly increased and then trended to decrease slightly.In the second part, the effect of treatment time on the growth characteristics, the Physical and chemical properties of MAO coatings respectively growth on the two kinds of material has been respectively studied in the optimized treatment solution. Under the premise of other fixed parameters, the effect of external resistance on the characteristic of MAO coatings has been comparatively studied. We have raised the equivalent circuit model of the process, and also preliminary analyzed the mechanism on the effect of the external resistance on the film properties. The results showed that: For the two kinds of material. as the processing time increased. the thickness of the coating. the micro-pores size. the surface roughness and the relative content of rutile TiO2 increased gradually, while the micro-pore density and the relative content of the anatase TiO2 decreased gradually. Within one hour. The MAO coating's growth process is respectively made up of two and three stages for the TA2 and TC4, as the processing time increased. The characteristics of the coating are strongly influenced by the external resistance. When the external resistance was added in the loop, the growth rate of the coating slowing down, but its growth time is extended:the relative content of rutile TiO2 is much lower than that without external resistance. but the coating has lower surface roughness and micro-pores size. and also the distribution and regular shape of its micro-pores are much better; Furthermore, the coating has fewer defects and micro-pores which pass through the coating. and also has lower porosity and greater corrosion resistance.In the third part, the TA2 commercially pure titanium and TC4 titanium alloy substrate had been respectively processed in the treatment solution containing different concentration of Al(OH)3 sol. The effect of the Al(OH)3 sol concentration on the thickness. phase structure. surface morphology. Micro hardness. corrosion resistance and electrochromic properties of the coating prepared on the two kinds of material have been detail studied. The results showed that:the Al(OH)3 sol concentration has great impact on the characteristics of the coating. With the increase of the concentration, the coating thickness and diffraction peaks of the rutile TiO2 are both gradually increased. After the concentration exceeding 40vol%. the coatings are all basically composed of rutile TiO2; The Micro hardness and corrosion resistance of the coating are gradually improved with the increase of the concentration, and then slightly decreases, when it exceeds 40vol%. The coatings respectively prepared on TA2 commercially pure titanium and TC4 titanium alloy at C=40vol% and 50vol%. which show a very stable injection and extraction of the charge and also demonstrate a regular variation after 600 times Cyclic voltammetry test. This phenomenon indicates that these coatings have good stability and coloration. After Cyclic voltammetry test, the coatings can still maintain a stable state and still closely bonded to the substrate.In summary, the coatings with excellent properties have been prepared on TA2 commercially pure titanium and TC4 titanium alloy substrate by using self-made multifunctional MAO preparation system. After microarc oxidation, the performance of the samples' corrosion resistance is greatly improved; furthermore, the coatings are closely combined with the substrate and have better mechanical properties. Above all. experimental data and results offer an important reference for the preparation of MAO coating with the more function, which also laid a solid foundation for us in the further research.